1. Field of the Invention
The present invention relates to exhaust gas recirculation (EGR) systems for internal combustion engines. More specifically, the invention is directed to EGR systems of the type which recirculate at least a portion of the engine exhaust gases into the engine air intake system for the purpose of reducing NOx emissions.
2. Description of Related Art
With continued tightening of governmental regulations on vehicular exhaust emission, particularly NOx, not only has the need to recirculate exhaust gases back to the engine intake become apparent, but so has the need to improve upon existing EGR technology.
U.S. Pat. No. 4,147,141 to Nagano discloses an EGR system which includes an EGR pipe for interconnecting an exhaust pipe and an intake pipe of an engine, an EGR cooler being positioned along the EGR pipe, a bypass pipe being arranged parallel to the EGR cooler, a selection valve for controlling the flow of exhaust gas through the EGR cooler bypass and an EGR valve mounted on the EGR pipe for controlling the flow of exhaust gas through the EGR pipe. The disclosed system operates to bypass exhaust gas around the EGR cooler under low exhaust gas temperature conditions while still providing exhaust gas recirculation, thus, avoiding overcooling of the gas and accumulation of carbon and condensation while reducing NO.sub.x emissions when exhaust temperatures are low.
However, because the EGR pipe of Nagano is closed by the EGR valve until engine load and engine temperature reach prescribed minimum values, no exhaust gas is recirculated, for example, under no or very low load, low engine temperature conditions, thereby increasing engine emissions under such circumstances. Furthermore, this disclosure is silent as to how this EGR system might be implemented on an engine having a supercharger or turbocharger compressor so as to avoid starving the turbine or diluting the charge and reducing the air/fuel ratio.
U.S. Pat. No. 5,203,311 to Hitomi et al. discloses an EGR system for recirculating exhaust gas from an exhaust passage through an exhaust gas cooler for return to an intake passage upstream of an engine driven supercharger and an intake air cooler to recirculate cooled exhaust gas for reducing No.sub.x. A bypass passage is provided to bypass exhaust gas around the cooler to the intake passage downstream of the supercharger during light load conditions to advantageously supply hot exhaust gas to the engine. A first control valve is positioned along the bypass passage to control flow therethrough and a second control valve is positioned downstream of the cooler to control the recirculation flow of exhaust gas. The bypass flow and cooled flow are directed into the intake passage on opposite sides of the supercharger, but another bypass passage may direct the cooled flow into the intake passage downstream of the supercharger. An embodiment which includes a turbocharger/compressor is also disclosed.
However, the Hitomi et al. patent fails to recognize the advantages of bypassing the cooler during start-up to reduce condensation in the piping and cooler, and thus, causes both control valves to be closed tinder no load and very light load conditions and to operate independent of engine operating temperature conditions. Furthermore, the Hitomi et al. patent fails to disclose an EGR system wherein the bypass flow and the cooled flow from the cooler are both directed into the intake passage on the upstream side of a supercharger or turbocharger. Moreover, the EGR system of this patent does not use a venturi, or a passive pump like an ejector, for directing the low pressure exhaust gas into the intake gases.
U.S. Pat. No. 5,205,265 to Kashiyama et al. discloses an EGR system for an internal combustion engine in which an exhaust gas recirculation pipe recirculates exhaust gas through a cooler into an intake pipe upstream of a supercharger and an intercooler wherein a bypass pipe branches off of the recirculation pipe to allow exhaust gas to bypass the EGR cooler during low load conditions. Under normal operating conditions the recirculated exhaust gas is directed back into the intake passage upstream of the supercharger. However, during low load conditions, the bypass pipe and a control valve are used to bypass gas back into an exhaust pipe and therefore the bypass flow is not returned to the intake flow during light load conditions, resulting in increased low load emissions.